target/riscv: Remove redundant assignment to SXL

SXL is initialized as env->misa_mxl which is also the mxl value.
So we can just remain it unchanged to keep it read-only.

Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230603134236.15719-4-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/csr.c

@@ -1321,10 +1321,6 @@ static RISCVException write_mstatus(CPURISCVState *env, int csrno,
 
     mstatus = (mstatus & ~mask) | (val & mask);
 
-    if (xl > MXL_RV32) {
-        /* SXL field is for now read only */
-        mstatus = set_field(mstatus, MSTATUS64_SXL, xl);
-    }
     env->mstatus = mstatus;
 
     /*